1. Field of the Invention
This invention relates to a method for purifying an exhaust gas and an apparatus therefore, and more specifically it relates to a method for purifying an exhaust gas containing catalyst poison(s) which comprises passing the gas, prior to its catalytic treatment, through a layer packed with a highly activated alumina to remove the catalyst poison(s) in the gas and an apparatus therefore.
2. Description of the Prior Art
Recently, there have widely been adopted methods of purifying an exhaust gas for prevention of air pollution or removal of offensive odor, which comprise catalytically treating the exhaust gas to remove harmful substance(s) in the gas which become(s) a cause of air pollution or offensive odor, for example, organic substance(s) contained in the exhaust gas from organic acid-production plants or aldehyde(s) contained in the exhaust gas from drying oven for paint coating and baking.
However, exhaust gases generally contain, besides harmful substances, dusts, tarry substances, which shall hereinafter be referred to as tar-like substances, organometal compounds, silicon compounds, phosphorus compounds, etc, which act as catalyst poisons to oxidation catalysts used for catalytic treatment of exhaust gases and become a cause to strikingly lower the performances and life of the catalyst. Particularly, organometal compounds, organosilicon compounds, organophosphorus, compounds, etc. have a striking catalyst poison effect. For example, organosilicon compounds and organophosphorus compounds are widely used as antifoamer in production of chemical products such as coatings and inks or additives for these products, or as flame retardants for as plastics, and hence, the exhaust gases from generation sources treating these products usually contain a trace amount of organosilicon compounds and organophosphorus compounds. These compounds in the exhaust gases oxidatively decompose on the oxidation catalysts for treatment of the exhaust gases and mask the active surface of the catalysts to strikingly lower the catalytic activity.
As methods for removal of such catalyst poisons, there have for example been proposed a pretreatment method using as an adsorbent active carbon, silica gel, activated alumina or zeolite in Japanese Laid-Open Patent Publication No. 138054/1977, and a pre-treatment method wherein the exhaust gas is in advance passed through a layer packed with alumina at a temperature of 150.degree. C. or more in Japanese Patent Publication No. 20333/1986.
However, it has been revealed according to studies of the present inventors that there are still problems both in the pre-treatment method using the above adsorbents and in the pre-treatment using the alumina-packed layer that the catalyst poisons cannot sufficiently be removed so that deterioration of the oxidation catalysts for treatment of the exhaust gases and shortening of life of the catalyst are brought about.
Further, in the pre-treatment method using the alumina-packed layer, since oxidative decomposition of the tar-like substances contained in the exhaust gases is inadequate, the tar-like substances are adsorbed in the alumina-packed layer and polymerized and further carbonized and deposited as carbon. As a result, removal of the above catalyst poisons, particularly organometal compounds, such as organosilicon compounds and organophosphorus compounds in the alumina-packed layer is strikingly reduced, and thus the effect of removal of the catalyst poisons by the alumina-packed layer is strikingly spoiled. Furthermore, since the carbon deposited on the alumina layer has a relatively lower ignition point, it abruptly ignites and burns when the temperature of the alumina-packed layer is raised, for example by heat transferred from the oxidation catalyst layer for treatment of the exhaust gases or by heat accumulation due to shutdown of the apparatus, etc. This causes various problems such as heat deterioration of the catalyst for exhaust gas treatment and damage of the apparatus, which are undesirable from the viewpoint of safety, too.
An object of the present invention is to solve the above problems by providing a method for purifying an exhaust gas whereby the organometal compounds such as organosilicon compounds, organophosphorus compounds and the like, which are catalyst poisons of the oxidation catalysts for exhaust gas treatment, as well as tar-like substances and the like are efficiently removed, and an apparatus for purifying the exhaust gas suitable for practice of the said method.